The Atmospheric Infrared Sounder (AIRS) data have revealed the low-level moisture preconditioning of the Madden-Julian Oscillation (MJO), but the cause of this preconditioning remains unknown. The present study addresses this issue by analyzing the latest satellite-based hydrological cycle data, i.e., moisture profiles from AIRS, rainfall from TRMM, surface evaporation from OAFlux, and total-column moisture transport from combined observations of surface wind vector by QuikSCAT, cloud drift wind vector by MISR and NOAA geostationary satellites, and precipitable water by SSM/I. Our analysis indicates that the low-level moisture preconditioning of the MJO is due primarily to the moisture convergence anomaly instead of surface evaporation anomaly associated with the MJO. Furthermore, the moisture convergence anomaly is mainly from the zonal rather than meridional component and should also be largely in the lower troposphere. These satellite-based results are consistent with the frictional wave-CISK theory in which the low-level zonal moisture convergence associated with convectively coupled equatorial waves plays a central role in organizing the low-level moisture preconditioning. The wind-evaporation feedback may be of secondary importance.